Decoupling of Two Closely Located Dipoles by a Single Passive Scatterer for Ultra-High Field MRI

We report decoupling of two closely located resonant dipole antennas dedicated for ultra-high field magnetic resonance imaging (MRI). We show that a scatterer slightly raised over the plane of antennas grants a sufficient decoupling even for antennas separated by very small gap (below 1/30 of the wavelength). We compare the operation of two decoupling scatterers. One of them is a shortcut resonant dipole and another is a split-loop resonator (SLR). Previously, we have shown that the SLR offers a wider operational band than the dipole and the same level of decoupling. However, it was so for an array in free space. The presence of the body phantom drastically changes the decoupling conditions. Moreover, the requirement to minimize the parasitic scattering from the decoupling element into the body makes the decoupling dipole much more advantageous compared to the SLR

Decoupling methods for radio frequency antennas in ultra-high-field MRI

For further development of many applications, such as multi-input multi-output (MIMO), radio frequency identification (RFID), wireless power transfer (WPT), and ultra-high-field magnetic resonance imaging (UHF MRI), decoupling of closely located antennas without significant changes of radiation pattern is a key task. In magnetic resonance imaging, there is a trend to increase the magnetic field strengths to obtain better spatial resolution of the image and perhaps smaller scan time. Since by increasing dc magnetic field, the frequency of the B1 field (created by dipole and loop antennas) necessary increases, the use of standard radio antennas applied in usual MRI is not allowed. Challenges of B1 field inhomogeneity arising at high frequencies and low efficiency of antennas in the new range are currently addressed by using so-called transmit-arrays driven by multiple independent channels and phase-amplitude steering. However, the mutual coupling results in considerable power losses and up to now poses a constraint on the density of these arrays. This dissertation focuses on developing new techniques for decoupling closely located dipole and loop antennas. In the first part, decoupling of closely located dipole antennas using passive scatterers is analyzed. The possibility of decoupling using passive scatterers and the effect of decoupling on the B1 field is comprehensively studied. In the second part, the decoupling of loop antennas is discussed. For that, shielded loop antennas with modified shields are analyzed. These shielded loop antennas suppress the mutual coupling between antennas without any damage on the B1 field. In general, using a metasurface of resonant dipoles, two closely located dipole antennas are decoupled while the presence of metasurface does not affect the B1 field significantly. Moreover, transceiver shielded loop antennas with self-decoupling feature are designed which does not change the B1 field. Besides ultra-high-field MRI application, the possibility of employing this method for wireless power transfer application has been numerically and experimentally checked

Dual-metasurface superlens: A comprehensive study

We present a theoretical and numerical study of a dual-metasurface superlens dedicated to the near-field optical imaging of submicron objects. Compared to the previous studies of dual-metasurface plasmonic superlenses, we suggest a more adequate theoretical model of their operation. The new model allows us to obtain twice better operational characteristics of the redesigned superlens. For the first time, we describe the operation of such the superlens using full-wave numerical simulations, taking into account the interfaces of the host medium slab and proving the nanoimaging for scattering objects instead of radiating sources. We discuss and address both application and fabrication issues for this superlens.Peer reviewe

Single Passive Scatter Decoupling Technique for Ultra-High Field Magnetic Resonance Imaging Application

| openaire: EC/H2020/736937/EU//M-CUBEIn this report, decoupling conditions between two dipole antennas, created by adding either a single passive dipole or single passive split-loop resonator (SLR), for ultra-high field magnetic resonance imaging (MRI) are compared. In contrast to our previously reported work, the decoupling granted by the dipole is advantageous. We numerically and experimentally demonstrate that parasitic impact of the passive dipole on distributed magnetic field inside the phantom is smaller than that of the passive SLR.Peer reviewe

Fluorescence quenching by plasmonic nanoantennas

Generalizing a previously developed analytical model of metal-enhanced fluorescence to the case of the strong coupling between a quantum emitter and a plasmonic nanoantenna, we study the fluorescence quenching in the strong coupling regime. When the nanoantenna is a simple Ag sphere and the quantum emitter approaches to its surface the fluorescence turns suppressed (both dipole and quadrupole moments of the system vanish) in the whole spectral range. However, if the nanoantenna is a plasmonic dimer with a tiny gap between two plasmonic nanoparticles, and the coupling grows due to the increase of the emitter dipole moment, the fluorescence quenching never occurs. This unexpected result explains why the nanolaser regime can be achieved with these nanoantennas, whereas a simple nanosphere coupled to quantum emitters can be a spaser.Peer reviewe

Dual-Band Transceiver High Impedance Coil Array for Ultra-high Field Magnetic Resonance Imaging

A novel approach for designing an element of an arrayed radio frequency coil for ultrahigh field magnetic resonance imaging (UHF MRI) is presented. The purpose of this approach is to achieve the dual-band transceiver regime in a compact array of magnetic antennas. Our work qualitatively develops the concept of the so-called high-impedance coil (HIC), combined with the adequate interfacing circuitry, which offers the decoupling of the HICs in the array due to the very high self-impedance compared to the mutual impedance. This concept has not been previously thought applicable to dual-band transceiver arrays. We show that, by slightly modifying only the cable shield, we achieve a dual-band transceiver high-impedance coil (dual-band TRHIC). The needed modification implies two asymmetric gaps granting two useful eigenmodes to the cable loop. The resonant excitation of these modes will allow the magnetic resonant scanning of both hydrogen and Phosphorus in the 7 T dc magnetic field. To verify our ideas, we simulated and measured a single TRHIC in both transmitting and receiving regimes and similarly studied two linear arrays made of two and three proposed TRHICs. The method of asymmetric gaps, in our opinion, allows one to engineer the targeted multifrequency operation of arrayed TRHICs decoupled at these frequencies.Peer reviewe

Double-resonant decoupling method in very dense dipole arrays

| openaire: EC/H2020/736937/EU//M-CUBEIn this paper an approach for broadening of operational band in a dense array of dipole antennas by implementing passive split-loop resonators (SLRs) as decouplers is presented. Compared to the previous method, where three closely located active dipoles were decoupled by two passive dipole, the operational band is significantly improved from 0.5% to 1.6% at the same level of decoupling −8 dB for the cross-talk and inter-channel transmittance. To delineate, the presence of two SLRs results in birefringence of the resonant interaction of SLRs which creates two different eigenmodes for decoupling. As a result, a dual-resonant decoupled band is obtained. Alongside with analytical investigation, numerical and experimental investigations verify the veracity of our approach. Moreover, the possibility of decoupling by SLRs for arrays with more active dipoles is investigated numerically.Peer reviewe

High-Impedance Wireless Power Transfer Transmitter Coils for Freely Positioning Receivers

In this letter, a wireless power transfer (WPT) system based on two high-impedance coil (HIC) - cable loop antennas with a modified shield - as transmitters (Tx)s and a spiral coil as a receiver (Rx) is proposed and discussed. Utilizing features of HIC at its parallel-circuit resonance frequency, we design the Tx in a way that in the absence of the Rx, the input impedance of the Tx is very high compared to the case when the Rx is near the Tx. This feature offers a possibility for free positioning of the Rx over an array of Txs and auto self-activation and de-activation of the Txs, leading to highly efficient performance. To verify the proposed solution, we have designed, fabricated, and experimentally tested a WPT system based on two HICs as Txs and one Rx. The proposed system operates in a high-frequency range (around 280 MHz) in the near-field coupling regime. The measured averaged efficiency of the prototype is higher than 93%. The proposed system is simple, cheap, and does not require any control circuit system for tuning the system when the receiver position changes.Peer reviewe